The manufacture and supply of high quality calcium carbonate for paper filler and for paper coatings is now widely practiced around the world. Relatively recently, particularly as alkaline papermaking has become popular, the on-site manufacture of precipitated calcium carbonate ("PCC") from aqueous solutions in atmospheric tanks has also been developed and implemented at a variety of locations. These on-site plants have developed because transportation costs of either a dry powder or of a liquid slurry of calcium carbonate was generally prohibitive. However, the variability of product quality from the heretofore-employed on-site PCC plants has been problematic at times. Such problems are especially acute in those locales where relatively impure sources of carbon dioxide have been employed, such as from boilers burning a variety of solid or liquid waste fuels. Also, the particle size distribution of the PCC obtained from various prior art processes has been less than optimum, and consequently, it would be advantageous to provide a process in which the particle size distribution could be more effectively controlled.
In processes employed for the manufacture of precipitated calcium carbonate, several fundamental chemical reaction steps are normally employed, which steps can be generally summarized as follows:
(1) Calcination--heating limestone (calcium carbonate) and driving the carbon dioxide out, resulting in the formation of lime (calcium oxide). PA1 (2) Slaking--reacting lime with water to form a lime slurry (calcium hydroxide; Ca(OH).sub.2); this reaction is accompanied by the evolution of heat. PA1 (3) Carbonation--reacting the lime slurry with carbon dioxide so that the solubilized calcium from the calcium hyroxide is reacted with the carbonate produced by bubbling the carbon dioxide in water, to form the desired calcium carbonate; this reaction is also exothermic. PA1 significantly increases the rate of the carbonation reaction and thus the production of precipitated calcium carbonate; PA1 significantly reduces the size of equipment and the building, thus reduces capital costs of on-site plants; PA1 increases the efficiency of carbon dioxide utilization, or carbonation efficiency; PA1 utilizes low concentration carbon dioxide sources, so that it can be effectively applied in a variety of locations where on-site precipitated calcium carbonate production has not heretofore been economically feasible; PA1 provides a low cost precipitated calcium carbonate; PA1 reduces the effect of fluctuations in CO2 concentration in flue gas and thus provides a high degree of particle size uniformity, to met optical quality requirements for use in paper manufacturing operations; PA1 provides a high quality precipitated calcium carbonate for filler in alkaline papermaking; PA1 provides a high degree of particle size uniformity, to meet optical quality requirements for use in paper manufacturing operations; PA1 enables the production of a variety of distinct crystal morphologies, including calcite scalenohedral, calcite rhombohedral, and aragonite; PA1 enables the efficient production of small calcium carbonate crystals; PA1 enables process control to be established using reliable and batch reproducible process parameters, thus enhancing quality assurance; PA1 enables the lime slaking production rate to be matched with the precipitated calcium carbonate production rate, thus significantly increasing operating rates and thereby reducing equipment size requirements;
Various prior art techniques disclose methods of preparing different PCC crystal morphologies, shapes, sizes, and size distribution of for the precipitated calcium carbonate. Although the prior art known to me teaches the use of process variables such as carbon dioxide concentration, calcium hydroxide concentration, temperature, and the use of chemical additives, none of such prior art processes known to me utilizes the step of carbonation under pressure, either alone or in combination with other heretofore utilized variables, as a technique for increasing the reaction rate, carbonation efficiency, or for making finer PCC particles. The prior art has also not employed pressurization of the carbonation reaction as a method for increasing the rate of formation of carbonate and calcium ions, the formation of which (and especially the latter) are the primary limitation in increasing the rate of carbonation reaction.
Moreover, the various prior art methods utilized for production of precipitated calcium carbonate in papermaking operations can be characterized in that the carbonation reaction has been carried out in an atmospheric pressure vented or open vessel. This means that the partial pressure of carbon dioxide available in the carbonation reactor has been limited based on the concentration of carbon dioxide available in an incoming gas stream.
It is in the carbonation reaction that the soluble calcium from the calcium hydroxide is converted to calcium carbonate. Then, more solubilization of the calcium ion takes place as the calcium hydroxide (lime slurry) is dissolved, and this proceeds until all of the available calcium hydroxide is converted into calcium carbonate. In this reaction, the reaction rate of calcium ions combining with carbonate ions is almost instantaneous. Consequently, the slow kinetic step which controls the overall reaction rate is believed to be the rate of dissolution of calcium hydroxide in the lime slurry, so that calcium ions are available for reaction. In conventional industrial processes for the manufacture of calcium carbonate, a slurry of approximately 200 gm/L of calcium hydroxide placed in an atmospheric reactor, and a gas containing from about 15% to about 20% by volume of carbon dioxide is bubbled through the slurry. In general, such prior art processes have a reaction rate such that calcium carbonate is formed at the rate of from about 0.5 grams per liter of slurry per minute to about 1.5 grams per liter of slurry per minute. Thus, for a batch charge of 200 grams per liter of calcium hydroxide, about 200 minutes is required to complete the reaction, per liter of slurry.
In general, the currently utilized manufacturing processes are slow, with low carbonation efficiencies. Thus, manufacturing plants utilizing such prior art processes require large equipment, resulting in high capital costs per unit of calcium carbonate production.
Relatively recently, approximately eighty percent (80%) of the world paper production has been converted to an alkaline papermaking process. In that process, precipitated calcium carbonate ("PCC") is employed as the primary filler. An average papermill may require from about 20,000 to about 100,000 tons per year of PCC. To meet such demands, the production of PCC has shifted from off-site to on-site. One important advantage of on-site PCC production has been the saving of transportation costs. Also, a primary raw material for PCC production, namely carbon dioxide, is available free at many mills, as a waste product from lime kiln flue gas. Such gas normally contains from about twelve percent to about twenty five percent (12%-25%) of carbon dioxide. However, one limitation encountered was that variability and fluctuation in the carbon dioxide concentration in the flue gas produced variability in the resulting PCC. Moreover, some mills do not have lime kilns, and free on-site sources of carbon dioxide are limited to flue gas from gas fired boilers, which only have seven to ten percent (7-10%) carbon dioxide concentration. In such situations, it has not heretofore been economical to place an "on-site" PCC plant at the mill location.
Thus, in order to manufacture large quantities calcium carbonate as required in papermaking operations, it has heretofore been necessary to provide very large reactors (for example, reactors in the 18,000 gallons to 20,000 gallons range are common). Thus it is evident that it would be desirable to provide a process in which the overall production rate of calcium carbonate is increased, thereby reducing the reactor size for a desired PCC production rate. It would also be advantageous to develop a process which (a) can utilize low CO2 containing gas, and (b) in which the effects of fluctuation in CO2 concentration on particle size distribution of PCC can be minimized.
Several prior art processes are known which superficially resemble portions of my process to some limited extent. In U.S. Pat. No. 3,304,154 issued on Feb. 14, 1967 to Dimitrios Kiouzes-Pezas for a Process for Producing Spheroidal Alkaline Earth Metal Carbonates, carbon dioxide gas is bubbled through a cylindrical autoclave reactor having a calcium hydroxide suspension therein. Pressure in the reactor was accumulated until a pressure from about 4 to 6 atmospheres gauge, and preferably about 5 atmospheres gage, was built up. Then, the reactor was rotated, while keeping the temperature between 60.degree. to 90.degree. Centigrade. However, that process has some practical limitations and thus is not well suited to the on-site production of PCC. First, it is difficult to produce the needed quantities (up to 100,000 tons per year) from such reactors, and starting at the low calcium hydroxide concentrations taught therein. Second, the process only produces spheroidal crystal structures. Finally, the rotation of the reactor presents various practical mechanical problems, and would result in undesirable cost and expense.
In U.S. Pat. No. 5,164,006 issued on Nov. 17, 1992 to Vasant Chapnerkar et al, for a Method for Preparing Acid Resistant Calcium Carbonate Pigments, gaseous carbon dioxide is added to a slurry of calcium hydroxide under atmospheric conditions. This conventional prior art process has a calculated reaction rate of approximately 1.0 grams per liter of slurry per minute, to produce a PCC product having a sclenohedral crystal habit with a surface area of 27,000 cm.sup.2 /gram (Blaine method). However, pressure carbonation was not utilized in that prior art process.
In U.S. Pat. No. 5,215,734 issued on Jun. 1, 1993 to Charles Kunesh et al, for Rhombohedral Calcium Carbonate and Accelerated Heat-Aging Process for the Production Thereof, a method of hydro-thermal post treatment of PCC is described. In that process, PCC produced under conventional process conditions is "heat aged" in a hydrothermal bomb at temperatures of up to 300.degree. C. for from 1 to about 24 hours, to cause the crystal structure to change to a rhombohedral PCC having a surface area of from about 3 to about 15 m.sup.2 /gram. So, this prior art technique uses conventional atmospheric PCC production, at relatively low reaction rates, before pressurization is utilized.
In summary, there continues to be a need for a high efficiency, simple method of production of PCC that is capable of efficiently producing large quantities of precipitated calcium carbonate. And, it would be advantageous to be able to employ such a process for on-site production of PCC at locations where only relatively dilute gas streams containing low percentages of carbon dioxide are available. Finally, it would be advantageous to employ such a process with flexible manufacturing capability, so that desired crystal shapes and sizes can be produced when and where required to meet the manufacturing requirements of a paper mill. Importantly, it would be desirable that PCC produced from a new method of on-site production of PCC would improve the properties of paper produced when utilizing the product from such a novel PCC manufacturing process.